It is well known in the art to supplement a friction braking system of a motor vehicle with a retarding system, thereby extending the useful life of the friction braking system. Larger motor vehicles often utilize retarders as a device to decrease the velocity of the vehicle without having to apply the friction brakes each time the vehicle slows.
There are several types of retarders that may be used to slow large motor vehicles, including retarders that restrict the flow of engine exhaust; those that modify the timing of one or more engine valves; and those that use the interaction between magnetic or electromagnetic forces. Each type of retarder functions to decrease the kinetic energy created by and contained in a moving motor vehicle. For the remainder of this document, unless specified otherwise, the term “retarder” will refer to a magnetic or electromagnetic type of retarder.
Magnetic and electromagnetic retarders typically include a shaft and a rotor that rotate together in a magnetic field. Retarding force is generated at the rotor by interactions between eddy currents created in the rotor and the magnetic field. Such systems are commonly called “eddy current retarders.” The retarding force created in the rotor is in the form of a drag torque that opposes the motion of a turning shaft or driveline.
The basic principles of electromagnetic retarders are well known. Generally, there are two types of electromagnetic retarders, those that use eddy currents to retard motion and those that use magnetic resistance. Eddy current retarders typically create a magnetic field and then pass a metallic component through the field. As the magnetic component passes through the field, free electrons in the metal move in circles as if caught in a whirlpool, called eddy currents. The eddy currents oppose the change that caused them (a reaction known as “Lenz's Law”), and therefore induced eddy currents will produce a retarding force when a metal component enters or leaves a magnetic field.
The rotational energy absorbed by the eddy currents in the rotor is converted to heat. Thus, cooling fins are typically added to the rotor to dissipate heat created by the eddy currents. Due to the large amount of heat created, adequate removal of heat from the system is required in order to prevent braking performance from deteriorating. This cooling process may be accomplished by air or fluid cooling. However, eddy currents are generally created in a rotating rotor or brake drum, and many difficulties result in sealing a rotating rotor or drum if fluid cooling is desired.
Electromagnetic eddy current retarders are well known in the art. For example, U.S. Pat. No. 6,176,355 (Jan. 23, 2001) to Yamamoto and assigned to Isuzu Motors Limited discloses an eddy current braking system for a vehicle. Generally, such eddy current electromagnetic retarders create the required eddy currents by either rotating a drum through a magnetic flux field or rotating a coil to generate a rotating flux field. The '355 patent, for example, uses eddy currents generated in a brake drum as the brake drum rotates through a magnetic flux field. U.S. Pat. No. 6,578,681 (Jun. 17, 2003) to Raad and assigned to Pacific Scientific Electrokinetics Division discloses an electromagnetic retarder in which the field windings that form the coil are mounted on and rotate with, the drive shaft.
Electromagnetic eddy current retarders typically include a stator that utilizes an electromagnet to generate a magnetic flux field. Stators are usually stationary while a rotatable rotor is connected to a torque delivery element, such as an axle or a driveline in a motor vehicle. The stator in electromagnetic eddy current retarders typically has coils composed of electromagnetic windings that may be excited with current supplied by a battery or an alternator system. Passing an electrical current through the coils creates a magnetic flux field. As the rotors pass through this field, a drag torque may be generated, thereby providing the desired retarding force.
Several difficulties may arise in the aforementioned retarder systems. Presently, eddy current retarders utilize what are typically known as brushes. Generally, brushes are the sliding connections that complete a circuit between a fixed and moving conductor. Due to their nature, brushes are often the source of failure of such systems.
In addition, eddy current retarders typically generate large amounts of rotational inertia. In retarder systems in which the majority of the system is rotating (the shaft, rotors and/or drum), this large amount of inertia works against the retarding force of the system itself and can reduce the effectiveness of the system.
In view of the foregoing, there is a need for an electromagnetic eddy current retarder that provides adequate retarding forces while effectively dissipating generated heat, eliminates the use of brushes, and reduces the rotational inertia of the system.
In at least some embodiments, the system and method of the present invention may provide advantages over known retarder systems. Some, but not necessarily all, embodiments of the present invention may utilize a fluid-cooled stationary drum and stationary coil. The stationary characteristic of these elements may reduce the difficulties commonly associated with fluid cooling. Some, but not necessarily all, embodiments of the present invention may eliminate the use of brushes by utilizing a stationary power source and a stationary coil, thereby increasing the reliability of the system. It is an additional advantage of some embodiments of the present invention to provide a retarder system wherein the only rotating element is that of a claw pole rotor, thereby decreasing the rotational inertia of the system. Additional advantages of various embodiments of the invention are set forth, in part, in the description that follows and, in part, will be apparent to one of ordinary skill in the art from the description and/or from the practice of the invention.